Re: [PATCH] Lazy FPU save/restore

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> The following patch saves the host FPU state and loads the guests FPU 
>> state if !(CR0.MP == 1 && CR0.TS == 1).
>>
>> When CR0.MP == 1 && CR0.TS == 1, all FPU activity will generate 
>> exceptions.  OS's use these exceptions to implement lazy FPU loading 
>> to improve context switch time.  Since any FPU activity will generate 
>> traps, we don't have to worry about the guest modifying the host FPU 
>> state.
>>
>> My microbenchmark of choice uses FPU operations so I think the 
>> results are currently tainted.  I've only tested on a 32bit SVM system.
>>
>> Signed-off-by: Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>
>>
>
> Applied, thanks.
>

... and reverted.

Running an fpu load on both guest and host shows corruption (both on the 
guest and host).  The load is


> #include <stdio.h>
>
> void test_fpu()
> {
>     int i;
>     double f = 0;
>
>     for (i = 0; i < 10000000; ++i)
>     f += 1 / (1.0 + i);
>     printf("%20.16f\n", f);
> }
>
> int main(int ac, char **av)
> {
>     while (1)
>     test_fpu();
> }

Compiled statically with -O2 on 64-bit.  I don't know what's wrong -- 
the code looks correct (well, on Intel it wants KVM_GUEST_CR0_MASK to 
include CR0_MP_MASK and CR0_TS_MASK, but it fails with that too, and it 
fails on AMD as well).

-- 
error compiling committee.c: too many arguments to function


-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV